Recently, there has been growing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, laptop computers, and even electric cars, efforts have increasingly been made towards the research and development of electrochemical batteries. In this aspect, electrochemical devices have attracted the most attention. The development of rechargeable secondary batteries has been the focus of particular interest. Many secondary batteries are currently available. Of these, lithium secondary batteries developed in the early 1990's are gaining attention due to their advantages of a higher operating voltage and a higher energy density than traditional batteries such as Ni-MH batteries.
Generally, lithium secondary batteries are fabricated by using a material allowing intercalation/deintercalation of lithium ions or alloying/dealloying for an anode and a cathode and filling an organic electrolyte solution or polymer electrolyte solution in between the anode and the cathode, and generates electrical energy by an oxidation/reduction reaction during intercalation/deintercalation of lithium ions in the cathode and the anode.
Currently, as an electrode active material (that is, a cathode active material) of a cathode of lithium secondary batteries, a cathode active material using nickel, manganese, cobalt, and the like, in particular, a lithium-manganese composite oxide cathode active material containing manganese (Mn) such as LiMn2O4 and LiMnO2 is gaining much attention for the reasons of lower production costs and less environmental pollution due to high capacity characteristics. However, a lithium secondary battery using a lithium-manganese composite oxide cathode active material has a drawback of a significant reduction in battery capacity during repeated charge/discharge cycles of the battery due to release of Mn ions at high temperature above about 40° C.
During battery discharging, Mn3+ ions in excess exist on a surface, and the capacity drastically reduces due to transition of Mn3+ from a cubic structure to a tetragonal structure by the Jahn-Teller effect. Also, Mn3+ undergoes a disproportionation reaction (2Mn3+->Mn4++Mn2+). In this disproportionation reaction, Mn4+ combines with lithium ions in an electrolyte to form electrochemically inactive Li2MnO3, and Mn2+ions are dissociated (dissolved) in an electrolyte solution, consequently, an amount of cathode active materials reduces. Thereby, the released manganese ions are electrodeposited in a form of metallic manganese on an anode surface, and block the movement of lithium ions, causing an increase in resistance, or act as a catalyst to cause reduction and decomposition of the electrolyte, thereby greatly reducing battery capacity and cycling and storage characteristics of the battery. Particularly, when overcharged at high temperature, the capacity reduces rapidly, and this is because a catalystic reaction is accelerated.
As a solution to this, attempts have been made to diffuse a lithium metal (in a shape of a plate or a foil) by a direct bond to a perforated current collector (for example, a foil), or to predope a lithium metal through a short between electrode active materials. However, use of the perforated current collector causes problems, for example, a reduction in a loading amount of electrode active materials, leading to capacity reduction, and a reduction in a contact area of the electrode active material with the current collector which, in turn, increases resistance to an electric current. Also, in the case of a certain cathode active material for high capacity, a problem occurs, for example, collapse of a crystal structure depending on a voltage region range, and resulting metal ions are known as degrading a solid electrolyte interface (SEI) layer generated on an anode surface.
Also, Japanese Patent Publication No. 7-153496 teaches that release of manganese ions in an electrolyte of a battery is prevented by adding at least one compound selected from the group consisting of BaO, MgO, and CaO to lithium manganese composite oxide. However, in practice, the above problem is difficult to solve sufficiently, and addition of an insulating compound causes a side effect of a reduction in initial output in the fabrication of a battery for high output.